This invention relates to an improved ground penetrating radar. In particular, it relates to a radar that has superior performance over known ground penetrating radar. The invention also includes an improved antenna useful with a ground penetrating radar.
The usefulness of a radar system capable of imaging subterranean features and buried objects has been recognised for almost thirty years. In this time various systems have been developed for a broad range of applications in the mining, geotechnical, environmental and safety areas. For example, applications include: detection of underground pipes and cables; detection of buried landmines and bombs; delineation of ore bodies; detection of aquifers; road evaluation; and hazardous waste detection. Two types of ground penetrating radar exist. For deep applications a borehole radar that propagates from a generally vertical antenna is preferred. For other applications a surface ground penetrating radar that propagates from a horizontal antenna is more suitable.
A useful discussion of borehole radars can be found in a paper by Sato et al published in Journal of Applied Geophysics 33 (1995) 53-61. The Sato paper discusses improvements in the performance of a borehole radar using cross-polarization measurements compared to co-polarization measurements. Although the Sato innovation was focussed on cylindrical antennas for borehole radars, the general discussion applies to ground penetrating radars in general.
A number of commercial ground penetrating radar systems exist which are based on conventional impulse radar transmitters with sampling head receivers. The performance of these systems is limited by the low mean transmitter power and inefficient receiver sampling.
An alternative to the impulse radar approach is to use a stepped frequency (or synthetic pulse) radar. One early example of this approach is described in U.S. Pat. No. 4,218,678, assigned to Ensco Inc. The invention related to a pulse radar system employing a synthetic pulse formed from a Fourier spectrum of frequencies generated and detected by digitally controlled transmitter and receiver circuits. The radar suffered from large amounts of cross-coupling between the antennas and large mismatch between the in-phase (I) and quadrature (Q) channels of the receiver circuit. These shortcomings caused ghosts and false images in the range profiles that limited the usefulness of the radar.
An improved version of the Ensco radar was developed by Xadar Corporation (a subsidiary of Ensco) and described in U.S. Pat. No. 4,504,833. The later radar employed a heterodyne receiver operating with a fixed intermediate frequency (IF) and incorporating a single frequency quadrature system. The radar suffered from excessive noise that limited the achievable resolution. The noise originated, in part, from discontinuities in the stepped waveform. A main disadvantage with the Xadar radar (and other continuous wave, CW, radar) is that strong signals either from leakage between the transmit and receive antennas or from shallow reflectors can mask weaker signals from deep reflectors. In other words, the useful dynamic range of CW radar is inadequate for many applications.
Another commercial system was developed by Coleman Research Corporation and described in U.S. Pat. No. 5,339,080. The Coleman patent acknowledges the Ensco system and claims to overcome the shortcomings. The radar system is very sophisticated and includes expensive radar and signal processing equipment. Cost reduction is unlikely as the hardware design dictates high power signal processing to obtain useful results. Furthermore, the radar is CW and therefore suffers the same dynamic range problems mentioned above. The Coleman radar also incorporates log spiral antennas that limit performance of the system because of the varying dispersive nature of the antennas with changing ground conditions.
U.S. Pat. No. 5,325,095, assigned to the United States Department of Energy, and U.S. Pat. No. 5,499,029, assigned to EGandG Energy Measurements Inc., describe a stepped frequency ground penetrating radar that incorporates a heterodyne receiver with a 500 kHz intermediate frequency. A quadraphase modulator modulates the transmitted signal so that in-phase and quadrature signals are transmitted alternatively. The reflected signal is mixed with a portion of the transmitted signal to obtain amplitude and phase information. The invention incorporates a log spiral antenna that limits the performance of the system, as mentioned above.
Despite development of a number of different stepped frequency ground penetrating radar, there is still scope for improvement in performance. In particular, improved penetration depth, greater resolution and reduced cost are necessary if ground penetrating radar are to be more usefully employed.
It is an object of the invention to provide a ground penetrating radar having improved performance compared to known ground penetrating radar.
It is a yet further object of the invention to provide an improved antenna for use with a ground penetrating radar.
It is a further object of the invention to provide an improved method of detecting subterranean features and buried objects.
Other objects of the invention will be evident from the following discussion.
In one form, although it need not be the only, or indeed the broadest form, the invention resides in a ground penetrating radar for detecting objects comprising:
signal generation means for
(i) generating and transmitting a stepped frequency master signal and
(ii) generating a tracking signal offset in frequency from the master signal by an intermediate frequency; return signal processing means for
(i) receiving a return signal reflected from the ground region and
(ii) processing the return signal using the tracking signal and the master signal to determine amplitude values and phase values indicative of target matter; gating means for
(i) pulsing the master signal and
(ii) gating the return signal; and antenna means for
(i) coupling the master signal into the ground and
(ii) coupling a reflected signal from the ground.
It will be appreciated that the ground penetrating radar may be of the surface or borehole type.
The signal generation means is suitably a dual frequency synthesiser that generates stepped frequencies in the range between DC and GHz with kHz resolution.
Preferably, the signal generation means also generates clock signals for synchronisation of operation of the return signal processing means and gating means.
The return signal processing means preferably includes a quadrature receiver and digital signal processing means. The quadrature receiver is suitably a dual channel receiver that mixes down a sample of the master signal and the return signal to the intermediate frequency using the tracking signal.
Suitably, the quadrature receiver incorporates mixer means, bandpass filter means, analog to digital conversion means and digital down-conversion means.
The digital signal processing means is suitably one or more processor means programmed with suitable signal processing algorithms.
The antenna means are suitably hollow elements. Preferably the elements are hollow pyramids with the feed applied between the apex. Antenna electronics are preferably located in one pyramid and are preferably self-contained.
Suitably the only link to and from the antenna means is a non-RF waveguiding link, such as a fibreoptic link.
In a further form the invention resides in an antenna means for a ground penetrating radar comprising:
a pair of hollow antenna elements, each antenna element having at least an apex, said hollow antenna elements being arranged such that said apexes are adjacent but not abutting;
electronics housed in one of said pair of hollow antenna elements; and
a coaxial connection between the apexes of said hollow antenna elements.
In a still further form the invention resides in a method of generating a radar signal for a ground penetrating radar including the steps of:
generating multiple narrower-band signals;
stepping said multiple narrower-band signals in frequency across an ultra-wide bandwidth;
generating a tracking signal for each narrower-band signal that is offset in frequency by an intermediate frequency from a centre frequency of said narrower-band signals; and
gating said multiple narrower-band signals.